1. Field of the Invention
The present invention relates to an apparatus for detecting plane positions of a substrate (a mask, reticle, semiconductor wafer, glass plate, and others which are used in the process of fabricating semiconductor devices or liquid crystal devices, for example). More particularly, the invention relates to an apparatus for positioning the surface of a substrate with respect to a given fiducial plane, a method of detecting plane positions preferably usable for a projection exposure apparatus (stepper, for example) which transfers the image of circuit patterns to a photosensitive substrate, for example, and an apparatus therefor.
2. Related Background Art
A plane detecting apparatus has hitherto been used widely in setting up proximity gaps, focusing, leveling, and the like, and often incorporated in an aligner which transfers to a given area on a photosensitive substrate the patterns on a mask or reticle (hereinafter referred to collectively as reticle) in the process, particularly in the lithography process, of fabricating semiconductor devices, liquid crystal devices, or the like. Especially in a stepper, when the reticle patterns are projected onto a photosensitive substrate (a wafer or a glass plate with photoresist being coated thereon) for exposure through an optical projection system having a high resolution, it is prerequisite to conduct an operation to match the surface of the photosensitive substrate exactly with the imaging plane of the reticle patterns, that is, a focusing adjustment.
In recent years, the focal depth of projection optical systems has increasingly become narrower and now it is only possible to obtain a focal depth of approximately .+-.0.7 .mu.m even with i rays having its wavelength of 365 nm as an exposure illuminating light. In addition, the projection field of the projection optical systems tends to have increasingly become greater year after year. Therefore, it becomes difficult to design and manufacture a projection system with which to obtain an extremely great focal depth over a wide exposure field (an angle of 22 mm, for example) entirely. In this respect, there have been proposed several methods whereby to expand the focal depth.
However, in order to attain a desirable focusing over a wide exposure field entirely, it is necessary to secure the flatness of an area on the photosensitive substrate locally within such an exposure field as well as the flatness of the imaging plane in any case. In other words, both the so-called curvature and inclination of the field should desirably be arranged. While the curvature and inclination of the field depend largely on the optical performance of the projection optical system itself, the flatness and parallelism of a reticle may cause them in some cases. On the other hand, the local area of the photosensitive substrate, that is, the flatness per stepper area (shot area) differs depending on the photosensitive substrate to be used, but it is possible to establish the parallelism between the surface of the shot area on the photosensitive substrate and the imaging plane by inclining the holder which holds the photosensitive substrate just minutely.
As a method to adjust focusing including a consideration given to the inclination of the surface of each one of shot areas on the photosensitive substrate such as described above, techniques are known as disclosed in U.S. Pat. No. 4,558,949, U.S. Pat. No. 4,383,757, and others, for example. Particularly, in U.S. Pat. No. 4,383,757, there is disclosed a technique wherein the spots of light beams are projected onto four points on a photosensitive substrate through a projection optical system in order to photoelectrically detect the spotted images by the reflective rays of light for the focusing adjustment as well as the calibration of the inclination (leveling) of the photosensitive substrate.
However, according to the two conventional techniques disclosed as above, the detection is made only to determine the displacement of the surface of the photosensitive substrate in the axial direction of the projection from a fiducial plane which is hypothetically set up (where it is matched with the imaging plane as much as possible). It is not a method to detect the displacement between the imaging plane and the surface of the photosensitive substrate directly. As a result, if the hypothetically established fiducial plane serving as the base on which to measure the positional displacement of the photosensitive substrate in the axial direction should be displaced from the imaging plane of the projection optical system due to drift or the like, such a displaced portion becomes a residual focus offset at the time of stepping the patterns to the photosensitive substrate.
In this respect, therefore, there are methods to reduce such a residual focus offset as disclosed in U.S. Pat. No. 4,650,983 and U.S. Pat. No. 4,629,313, for example. In the U.S. Pat. No. 4,650,983, a fiducial pattern is arranged on a stage (holder) to stack a photosensitive substrate thereon, and this fiducial pattern is inversely projected on a specific pattern on a reticle through a projection optical system. Then, the height of the stage is adjusted so that the contrast of the image of the fiducial pattern to be formed on the specific pattern becomes the greatest. Subsequently, a focus detection system (inclined incident type) for the focus adjustment of the photosensitive substrate is calibrated so that the surface where the fiducial pattern is formed can be detected as the best focus plane (the optimal imaging plane). Also, in the U.S. Pat. No. 4,629,313, a fiducial pattern on a stage serves as a slit type photosensor, and the best focus plane is specified by detecting the contrast of the pattern images formed by a projection optical system when the slit pattern on a reticle is projected.
Also, as another method, there is known a technique as disclosed in U.S. Pat. No. 4,952,815 wherein a luminescent mark which emits slit rays of light is arranged on a stage, and the image of this luminescent mark is inversely projected onto a specific mark on a reticle. Then, the stage is shifted in the XY directions, and the best focus plane is detected by photoelectrically detecting the light which is transmitted above the reticle when the reticle mark is scanned with the image of the luminescent mark.
Also, as a method which is developed from the inclined incident type focus detection system disclosed in the U.S. Pat. No. 4,558,949, there is known a multipoint inclined incident type focus detection system as disclosed in U.S. Pat. No. 5,118,957, for example, wherein a pinhole image is projected by an inclined incident system onto each of plural points (five points, for example) in a shot area on a photosensitive substrate, respectively, without any intervention of projection optical systems, and each of the reflective images therefrom is collectively received by a two dimensional position detecting device (CCD). This method disclosed as a prior art is the so-called multipoint AF system of an inclined incident type whereby to implement both a highly precise focus detection and inclination detection. However, there is no disclosure or suggestion in this prior art at all as to the calibration of the focus offset with respect to the best focus plane at the time of stepping.
In each of the above-mentioned prior arts, any one of the systems to detect the positional displacement (focus displacement) in the axial direction of the projection light on the surface of a photosensitive substrate at the time of actual pattern alignment is simply to detect only the positional displacement of the photosensitive substrate in the axial direction and not to detect any focusing state of the reticle patterns and the photosensitive substrate directly. Ideally, therefore, calibration is conducted occasionally in consideration of drift and the like on the apparatus. Furthermore, with the flatness and parallelism of the photosensitive substrate taken into consideration, the multipoint AF system is superior because it performs the focus detections at plural points in a shot area individually and almost simultaneously.
Nevertheless, when a multipoint AF system is employed and a calibration is needed for such an AF system, it is found difficult to apply any of the conventional methods of calibration as it is. In other words, for the conventional calibration method, it is always necessary to detect a specific pattern on the reticle by some detection system. Accordingly, it is required to inscribe such a specific pattern in the circumference of the circuit pattern area or in the street line area of the target reticle at all times. As a result, the plural positions of the measuring points in the projection field determined by the multipoint AF system are completely different from the position of the specific pattern on the reticle which is to be detected at the time of calibration. Naturally, there is a problem that any accurate calibration is possible by the calibration as it is due to the aberration of the projection optical system, warping of reticle, or the like unless such a difference in the measuring position is taken into account. Besides the multipoint AF systems, the same problem is encountered in using a fixed point AF system in which the focus measurement point on the photosensitive substrate for the inclined incident method is set in the center of its projection field, that is, only one point in the center of the shot area.
Here, in FIG. 34, there is illustrated the conventional structure of a focus detecting system (plane position detecting system) of an inclined incident type in which the plane position of one shot area on a photosensitive substrate is detected as an amount of deviation with respect to the fiducial plane (the imaging plane of a projection optical system, for example). The focus detecting system shown in FIG. 34 is equivalent to the fixed AF system which has been described above. In FIG. 34, a projection optical system PL projects the reticle pattern to be imaged on a wafer W. The pattern image of the reticle is formed in the optimal imaging plane (best focus plane) which is perpendicular to the optical axis AX in a state where the contrast becomes greatest. A Z stage 20 with the wafer W stacked thereon is minutely moved on an XY stage 21 in the axial direction AX (in the direction Z) to enable the surface of a specific shot area on the wafer W to be matched with the best focus plane.
Now, in order to detect the height position of the surface of the wafer W, that is, the deviation amount of the shot area surface in the direction Z with respect to the best focus plane, there are provided a light projector LSU, an imaging lens system L.sub.1 for the light projector, an imaging lens system L.sub.2 for a light receiver, and the light receiver RVU as the focus detection system of an inclined incident type. The light projector LSU projects imaging beams onto the surface of the wafer W in the diagonal direction through the lens system L.sub.1, and the light receiver RVU receives the positively reflective beams from the wafer W through the lens system L.sub.2. Then, the light receiver RVU outputs to a focus error detecting circuit FD the photoelectric signals which vary in accordance with the positions where the reflective beams are received. Usually, the imaging beams from the light projector LSU are projected to the vicinity of the position where the optical axis AX of the projection optical system PL exists, and the arrangement is made so that the surface of the wafer W (a local plane where the beams from the light projector LSU are being projected, to be exact) is allowed to match the best focus plane when the positively reflected beams are received to match the detection center of the light receiver RVU.
Also, the error detecting circuit FD calculates the level deviation signal proportional to the positional deviation amount of the wafer surface in the direction Z with respect to the best focus plane on the basis of the signal from the light receiver RVU, and transmits it to a driving portion (hereinafter referred to as Z-DRV) 18 of the Z stage 20. The Z-DRV 18 servo controls the Z stage 20 in order to make the level of the deviation signal a predetermined target value (zero or a given value). Here, in this case, there is provided in the Z-DRV 18, a circuit to detect the difference between the deviation signal level and the target value and determine whether such a difference is within an allowable range or not. This circuit is needed to define the allowable range significantly narrow against the target value thereby to make it possible to implement a stabilized servo control at a high speed.
In the apparatus shown in FIG. 34, the level variation of the deviation signals output from the focus error detecting circuit FD is proportional to the positional variation of the surface of the wafer W in the direction Z in the vicinal range of the best focus plane. Its proportional constant corresponds to the detection sensitivity for plane positions by the focus detection system of the inclined incident type shown in FIG. 34, and given the positional deviation amount of the wafer in the direction Z as .DELTA.Z (.mu.m) and the amount of the level variation of the deviation signal as .DELTA.V in terms of a voltage at that time, the detection sensitivity (inclination) can be defined as .DELTA.V/.DELTA.Z. The greater the value .DELTA.V/.DELTA.Z, the higher becomes the detection sensitivity. Thus, the response of the Z stage 20 is enhanced. On the other hand, however, the stability of the servo system may be affected. Also, a problem is encountered that if the detection sensitivity is lowered on the contrary, the accuracy of the servo system pursuance is reduced.
In a servo system of the kind, the response, stability, and pursuance accuracy are optimally set, but the detection sensitivity itself is not necessarily uniform among focus detection systems when a plurality of aligners are examined. In other words, it often depends on an individual apparatus. Therefore, even if the allowable range against a target value at the time of a servo control is set equal in terms of the deviation signal level, the amplitude of the actual allowable range for the positional deviation in the direction Z may differ per apparatus due to difference in detection sensitivity (inclination and rate of change). Consequently, even when the result of exposure is desirable after the focus adjustment by a certain aligner, there may occur a problem that the conditions obtainable from the focusing parameters at that time are not reproduced as they are in another aligner. This presents a serious problem in terms of the accuracy management in fabricating devices in the manufacturing lines using a number of aligners.
The description has been made of a fixed point AF system so far, but when the foregoing multipoint AF system is employed, it is necessary to output deviation signals by the focus error detecting circuit for each of many numbers of the measuring points which are set in the shot area. Therefore, unless the detection sensitivity (.DELTA.V/.DELTA.Z) is uniform at each of the measuring points, the allowable positional deviation range for the servo system control at each of the measuring points becomes different. A problem is likewise encountered that the advantages obtainable in using the multipoint AF system are reduced by half.
Also, when a step exists in one shot area, it is conceivable to detect the height positions at the upper and lower portions of such a step using a multipoint AF system and set up the surface of the shot area to the fiducial plane in accordance with an averaged value of the respective results of the detections. However, it is impossible for this method to detect the height positions at one point of the plural measuring points, or if the detection results are questionable (the detection errors are great), there is encountered a problem that it is impossible to obtain the height position of the entire shot area exactly. Now, it is obvious that the focal depth of the projection optical system has increasingly become smaller year after year and that the step in the shot area has been greater on the contrary. For any of steppers, this is an extremely serious problem because it brings about another problem that the shot area cannot be set up accurately to the fiducial plane over all. Moreover, although it is possible to obtain the inclination of the shot area surface with respect to the fiducial plane from the detection results of the multipoint AF system, the error component in the amount of such inclination is great if only the shot area inclination is obtained simply from the set up positions of the plural measuring points in the shot area and the height positions of the respective measuring points when any step exists in the shot area as described above. Hence, there is a problem that the surface of the shot area cannot be set up exactly in parallel to the fiducial plane.
Also, with a fixed point AF system and a multipoint AF system, a levelling sensor of the collimator type as disclosed in U.S. Pat. No. 4,558,949 is in some cases provided in a projection exposure apparatus. This levelling sensor, however, only detects how much the surface of a wafer is inclined with respect to an imaginarily preset fiducial plane, and is not of a type which directly detects the inclination of the surface of the wafer with respect to the imaging plane of a projection optical system. Therefore, if the imaginary fiducial plane and the imaging plane deviate from each other, the amount of that deviation will become residual levelling offset as in the aforedescribed multipoint AF system. This residual levelling offset can be reduced by finding the imaging plane of the projection optical system by the use of the technique disclosed in U.S. Pat. No. 4,650,983, U.S. Pat. No. 4,629,313 or U.S. Pat. No. 4,952,815, and calibrating the levelling sensor so that the imaging plane may coincide with the imaginary fiducial plane. However, to find the imaging plane of the projection optical system accurately in the above-described technique, use must be made of a reticle exclusively for measurement having a number of particular marks (slit patterns). This leads to a problem that much time is required for the calibration of the levelling sensor and the throughput of the apparatus is greatly reduced.
Further, in the projection exposure apparatus, the position of a particular mark on the reticle is detected by the use of a light emitting mark provided on a stage, to thereby effect reticle alignment, the measurement of a base line or the measurement of the imaging characteristics (such as projection magnification and distortion) of the projection optical system. An apparatus of this type is disclosed in U.S. Pat. No. 4,853,745, wherein light transmitted through a reticle is directed to a photoelectric detector through a bending mirror disposed above a particular mark or a beam splitter disposed in an illuminating optical system. However, when the positions of a plurality of particular marks on the reticle are to be detected, it is necessary to dispose the bending mirror and the photoelectric detector relative to each particular mark or make the bending mirror and the photoelectric detector movable, and this leads to a problem that the apparatus becomes bulky. Also, the beam splitter disposed in the illuminating optical system gives rise to a problem that the illuminating optical system becomes bulky and complicated.